Various countries have environmental regulations for vehicles which limit emissions of certain compounds, such as nitrogen oxide. For example, some regulations require that many newly-manufactured diesel-powered vehicle engines significantly reduce nitrogen oxide levels. One technology addressing this concern is selective catalytic reduction (SCR), which involves dosing a reductant into engine exhaust upstream of a catalyst to convert nitrogen oxides into less harmful byproducts. Diesel exhaust fluid (DEF) is a generic term for a reductant that may be used in the process of SCR. An example of a common reductant is a 32.5% solution of aqueous urea.
Because many manufacturers have adopted SCR technology, SCR systems will often be installed on new diesel vehicles. Correspondingly, diesel vehicles may now incorporate special DEF tanks, and DEF dispensers are increasingly provided in retail service station environments.
However, DEF will crystallize and freeze at a relatively high temperature (approximately 12° F.) compared to liquid fuels such as gasoline. In addition, DEF expands approximately 7% when frozen. This expansion can cause damage to the internal components of a DEF dispenser.
One prior art solution to this problem involves mounting a 750 W/120V electric heater in a DEF dispenser's lower hydraulic cabinet adapted to turn on when the ambient temperature in the cabinet reaches a specified level (e.g., 41° F.). Likewise, the solution may involve providing DEF dispensers with a retractable dispensing hose that is stowed in the dispenser's cabinet and a sliding cover or access door over the dispenser nozzle. Alternatively, the DEF dispenser may be adapted to suspend operation if the ambient temperature in the hydraulic cabinet reaches 12° F. while the power is energized to prevent damage to the dispenser's fuel handling components.
Temperature effects have also presented problems in prior art liquid fuel dispensers. Liquid fuel dispensers are well known, and these dispensers include flow meters that measure volumetric flow rate of liquid fuel as it is dispensed. Such flow meters are typically required to comply with weights and measures regulatory requirements that mandate a high level of accuracy. This ensures that the customer is neither overcharged nor undercharged for the purchase. Typically, either positive displacement meters or inferential meters have been used for this purpose.
The volume of liquid fuel is somewhat dependent on temperature (i.e., it expands when heated and contracts when cooled). In addition, liquid fuels are typically sold by a volumetric measure, such as U.S. gallons. Prior art solutions provide temperature compensation by sending signals from thermometric probes located in a flow meter to a first circuit in the dispenser's lower fuel handling compartment, to a second circuit in the dispenser's upper electronics compartment via an intrinsically safe connection, and finally to a computation device designed to combine the temperature data and pulser data. The computation device employs a volume correction factor to compensate the pulser data so as to account for temperature variations. Detailed information regarding temperature compensation of dispensed fuel is disclosed in U.S. Pat. No. 5,557,084 to Myers et al., entitled “Temperature Compensating Fuel Dispenser,” the entire disclosure of which is incorporated herein by reference for all purposes. However, this solution may not be available in many markets due to government regulation.